Chemistry Reference
In-Depth Information
Figure 9.5
Connecting microfluidic devices to the world: an image of an actual device. Microfluidic device for
combinatorial chemistry integrated to an eight-valve manifold through tygon tubing. Reprinted from [47] with
kind permission from Springer Science
+
Business Media. © 2009.
microchip is surrounded by bulky valves and a bundle of tubing needed for the fluid supply (pumps),
actuation (by gas) and detection [47]. One can find examples where there are dozens of connecting tubes,
indicating the number of actuating (perhaps syringe-type) pumps that are required. The supporting equipment
is usually not shown in illustrations of microfluidic devices. Although microchips can perform many
sophisticated operations, the analytical process could only be considered truly microminiaturized if the
instrumentation supporting the unit operations in the chip could be miniaturized by many orders of magnitude
as well.
The problem of the so-called 'world-to-chip' interface has plagued microfluidics since its inception.
Microfluidics must integrate all components of the system on the same chip to ensure the portability and
minimum energy consumption required by the principles of Green Analytical Chemistry. Pumps, valves,
mixers, and so on must be miniaturized in order to achieve an integrated system, which forces a choice
between active methods - efficient, but requiring energy and difficult to miniaturize - and the passive methods
provided by non-instrumental microfluidics - easier to integrate, but less efficient. This is a huge challenge,
especially in biotechnology, where the volume of the targets of study located in the macroscopic environment
must be reduced to accommodate the microscopic environment of the microfluidic device. Finally, the huge
surface/volume ratio of miniaturized systems could modify the physical behaviour of the system and create
new problems, such as target molecules adhering to the solid walls, or the fluid being prevented from entering
the microchannels due to the effect of capillary forces. According to some researchers, the dilemma of the
world-to-chip interface is one of the bottle-necks in the development of
TAS [48]. Solving this problem is
critical for high-throughput applications where manual manipulation is not economical and a macro-to-micro
interface must be developed.
Nevertheless, the main thrust of portable CE systems development is towards chip-based miniaturized CE.
However, capillary-based (non-chip) portable CE systems have certain unmatched advantages as recently
described by Ryvolová
et al.
[49]. These include the relatively simple cylindrical geometry of the CE capillary,
maximum volume-to-surface ratio, no requirement to design and fabricate a chip, the low costs of capillary
compared to chip, and better performance with some detection techniques. On the other hand, microfluidic
chips - the key components of LOC devices - have frequently been designed for very specific applications.
They are relatively expensive and unique. Compared to portable field analysers based on 'classical' CE, in
which a capillary can easily be discarded if a problem occurs, custom-made microfluidic chips are definitely
not yet disposal products. This condition restricts their use in field applications.
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